1. Field of the invention
This invention relates to a method of preparing ceramic-ceramic composite powders in which ceramic components of a certain type are combined with other ceramic components, and metal-ceramic composite powders in which metal components are combined with ceramic components, and to the ceramic composite powders obtained thereby. More particularly, it relates to a method of enabling preparation of the composite powders in which any arbitrary amount of metal oxides or metal is supported on the surface of an inorganic powder.
2. Description of the Related Art
One typical method of synthesizing ceramic products includes the steps of weighing ingredients of a ceramic starting material in the respective amounts to obtain the predetermined composition, thoroughly mixing the components to obtain uniform distribution, and calcining the resultant mixture. According to this method, the calcined product must be pulverized and subjected to repeated mixing and calcining to obtain a sufficiently homogeneous composition.
Metal powders or ultrafine metal particles have conventionally been used extensively as conductive or magnetic materials without being supported by any particular powder. The prior art metal-ceramic composite materials such as oxide dispersion strengthened alloys, heat resisting structural materials and damping materials demonstrate their functions by dispersing the ceramic particles in the metallic matrix.
According to the above conventional method of synthesizing ceramic products, despite thorough mixing, it is difficult to obtain an ideally homogeneous composition of particles the diameter of which is in the order of microns, because of aggregation of the component powders. In order to obtain a ceramic product having an ideally homogeneous composition, ceramic-ceramic powders having the predetermined composition should be used. However, no method has yet been found for synthesizing such ceramic-ceramic composite powders as are suitable for the purpose.
Use of metal powders or ultrafine metal particles not supported on any special powder is disadvantageous in that (1) their specific gravity is relatively high, (2) they are hard to disperse, (3) they are hard to handle, and (4) metal particles are easily sintered when exposed to heat. For example, when a metal powder is mixed with an organic binder to obtain a conductive paste, the metal powder tends to become separated from the organic binder because the metal powder has a greater specific gravity than the organic binder. Moreover, when the product is to be used as a paint in the form of powder rather than in flakes, the resultant coating layer tends to become uneven.
In order to solve these problems, use of composite powders in which inorganic powders are coated with metal by means of electroless plating has been proposed. However, the method is detrimental because electroless plating is expensive and requires a highly complicated process.
Also, the metal-ceramic composite materials such as oxide dispersion strengthened alloys, heat resisting structural materials and damping materials can be obtained by mixing a ceramic powder with a metal powder and sintering the resultant mixture. Because of the difference in the specific gravity between the ceramic powder and the metal powder, it is extremely difficult to uniformly disperse the ceramic powder in the metal powder. Metal-ceramic composite materials in which the ceramic powder is uniformly dispersed in the metal powder can be obtained, provided that metal-ceramic composite powders in which metallic and ceramic components have been combined are sintered. As mentioned above, such metal-ceramic composite powders can be obtained by the electroless plating method, which is however defectively expensive and complicated in its process.
The present inventors have completed this invention in the course of their studies on inorganic ion exchangers by noting that ceramic-ceramic composite powders in which metal oxides are uniformly supported on the surface of an inorganic powder can be obtained by precipitating metal hydroxides, metal basic salts, or metallic salts uniformly on the surface of the inorganic ion exchanger and then calcining the resultant composite powders, and that metal-ceramic composite powders in which the inorganic powder is uniformly coated with the metal, can be obtained by reducing the metal oxides to metal.